In an OFDM system, each sub-data stream has a relatively low bit rate by decomposing a data stream into several sub-data streams through the OFDM, and then each sub-data stream is respectively modulated on the corresponding sub-carrier for parallel transmission. It should be pointed out that, various sub-carriers of OFDM are not only orthogonal with each other, but also have an overlap of ½.
As information symbols of a single-carrier system are directly modulated on the time domain (or are some simple variations), the Peak Average Power Ratio (PAPR) is relatively low. For the multiple-carrier system, as at the same time a plurality of carriers transmits the information symbols simultaneously, while the information symbols carried by each carrier are independent with each other, the PAPR is relatively large, which is generally 2-3 dB larger than the single-carrier system.
In the Long Term Evolution (LTE) system, the PAPR problem of the User Terminal (UT) is sufficiently considered. The OFDM system has a larger PAPR, that is, the instantaneous value of the output signal of the transmitter will have greater fluctuations, which will require some components within the system, such as a power amplifier, Analog-Digital (A/D) and Digital-Analog (D/A) converters etc., to have a large linear dynamic range. At the same time, the non-linear of these components will generate a non-linear distortion on the signal with a large dynamic range, and the generated harmonics will cause mutual interference on sub-channels, thus affecting the performance of the OFDM system. Therefore, in the LTE system, upstream multiple-access selects Single Carrier-Frequency Division Multiplex Access (SC-FDMA) finally, a very important reason of which is the PAPR problem. The high PAPR increases the requirement on the linearity of the power amplifier, which is particularly disadvantageous to the UT, so a relatively ideal solution of the upstream multiple-access is a single-carrier system with a cyclic prefix, i.e., SC-FDMA.
At present, the research on the multiple access on the basis of the OFDM system is a hot point. However, the researches on a plurality of multiple access modes such as Orthogonal Frequency Division Multiple Access (OFDMA) and SC-FDMA and so on, which exist simultaneously, are few. Take the LTE system for an example (at present, in the system, there is only one multiple access mode for the upstream and downstream respectively), at present, the system uses the OFMDA on the downstream, and uses the SC-FDMA on the upstream, but cannot well suitable for requirements of new systems. The LTE-Advanced system and the International Mobile Telecommunications-Advanced (IMT-Advanced) system put forward relatively high requirements on the peak data rate and spectral efficiency, which necessarily leads to reconsider the application of the OFDM system.